Although the most commonly used semiconductor memory at the present time is the dynamic random access memory (DRAM), static random access memories (SRAMs) find many applications because of, e.g., their relatively high speed as compared to DRAMs. A typical SRAM cell has four transistors and two load resistors. The resistors may be formed by a field-effect transistor in its stare or by high-resistance polysilicon. To reduce the substrate surface area used by the cell, the load resistors are typically above and separated from the four transistors by an intervening dielectric layer. A dielectric layer is deposited on the load resistors and further processing forms the contacts to the resistors, bit lines, etc.
The load resistors should satisfy several criteria, including high resistance and comparable values; that is, the resistors in a cell should be balanced so as to have approximately equal resistances. However, we have found that in SRAM cells with polysilicon resistors the values of the resistances can vary enormously; for example, from 1000 Teraohm/square to 1 Mohm/square. Not only can the resistors be unbalanced, but they may have values that are unacceptably low.